<svg width="357" height="267" xmlns="http://www.w3.org/2000/svg">
 <!-- Created with SVG-edit - http://svg-edit.googlecode.com/ -->
 <g>
  <title>Layer 1</title>
  <path id="svg_4" d="m80.14841,179.802l23.5,-56.7l26,-26.0004l42.9996,-8l41,9l26,22.0004l33,61.00001" stroke-width="5" stroke="#000000" fill="none"/>
  <line id="svg_1" y2="200.10235" x2="175.14844" y1="77.10235" x1="175.14844" stroke-width="5" stroke="#000000" fill="none"/>
  <line id="svg_3" y2="137.10077" x2="345.14844" y1="138.10077" x1="11.14844" stroke-width="5" stroke="#000000" fill="none"/>
  <text xml:space="preserve" text-anchor="middle" font-family="serif" font-size="24" id="svg_10" y="70.1024" x="174.148" stroke-width="0" stroke="#0000ff" fill="#000000">dn/dt</text>
  <text xml:space="preserve" text-anchor="middle" font-family="serif" font-size="24" id="svg_12" y="162.103" x="344.148" stroke-width="0" stroke="#0000ff" fill="#000000">n</text>
  <text fill="#000000" stroke="#0000ff" stroke-width="0" x="176.148" y="224.103" id="svg_11" font-size="24" font-family="serif" text-anchor="middle" xml:space="preserve">n=0</text>
  <ellipse fill="#ff0000" stroke="#000000" stroke-width="5" cx="136.59943" cy="138.2" id="svg_2" rx="48.5" ry="9"/>
  <g id="svg_9">
   <path transform="rotate(-180, 294.398, 136.752)" id="svg_7" d="m285.14847,121.10216l18.5,14.21323l-17.31915,17.08694" stroke-width="5" stroke="#007f00" fill="#007f00"/>
   <rect id="svg_8" height="6" width="26" y="133.10203" x="301.14844" stroke-width="5" stroke="#007f00" fill="#007f00"/>
  </g>
  <ellipse id="svg_16" fill="#007f00" stroke="#000000" stroke-width="5" cx="213.59943" cy="137.2" rx="48.5" ry="9"/>
 </g>
</svg>